In 2P, it's worded that you can play Engineers (Allies) or Bridge Engineers (Germans) to (emphasis mine) "build and repair bridges". The use of the plural implies (to me) that there is no limit other than number of cards in hand.
In GS, it's a little more confusing. On p 13 of the rules, it says "You may play a Bridge Engineers command event from your hand to repair and build bridges", but in the sequence of play on the back of the rulebook it says "You may play a Bridge Engineers command event to repair or build a bridge". The p13 reference is especially confusing since it mentions playing "a Bridge Engineers command event" but then uses "bridges". I tend to think that it's intended to be a single bridge repair in GS.
Finally, there's no repair/building of bridges mentioned in the AS Supply Phase.
re GS I interpreted the wording on the back of the rulebook "You may play a Bridge Engineers command event to repair or build a bridge" to mean that a single card can repair/build one bridge (and not 2+ bridges). i.e. that if you play two cards you can repair/build two bridges, and so on. This interpretation would therefore not conflict with the rule on P13 (I think...)